Flight delays happen often and can certainly put a damper on the beginning or end of an otherwise perfect vacation. Based on a review of data compiled by FlightStats, which tracks monthly flight data by airline, 24/7 Wall St. has identified the airlines with the most delays.

When it comes to flight delays, not all air carriers are not created equally. Airlines have very different track records when it comes to delays because of their travel routes, weather and other factors. Delays can cause travelers to miss a connecting flight, a scheduled business meeting or simply eat away at their priceless vacation time – despite the high ticket prices travelers paid for the promised service.

Flight problems account for the biggest percentage of problems involving air travel, which are divided into three categories – flight delays, missed connections and cancellations.

To identify the airlines with the most delays, 24/7 Wall St. reviewed the share of total flights that were delayed for at least 15 minutes in July by airline. Only major airlines were considered. They also considered the total number of flights that were cancelled and the average length of delays in July. All data came from airline data website Flightstats.